Altering gene expression through chromatin modification can be accomplished by inhibiting histone deacetylase (HDAC) enzymes. There is evidence that histone acetylation and deacetylation are mechanisms by which transcriptional regulation in a cell—a major event in cell differentiation, proliferation, and apoptosis—is achieved. It has been hypothesized that these effects occur through changes in the structure of chromatin by altering the affinity of histone proteins for coiled DNA in the nucleosome. Hypoacetylation of histone proteins is believed to increase the interaction of the histone with the DNA phosphate backbone. Tighter binding between the histone protein and DNA can render the DNA inaccessible to transcriptional regulatory elements and machinery. HDACs have been shown to catalyze the removal of acetyl groups from the epsilon-amino groups of lysine residues present within the N-terminal extension of core histones, thereby leading to hypoacetylation of the histones and blocking of the transcriptional machinery and regulatory elements.
Inhibition of HDAC, therefore can lead to histone deacetylase-mediated transcriptional derepression of tumor suppressor genes. For example, cells treated in culture with HDAC inhibitors have shown a consistent induction of the kinase inhibitor p21, which plays an important role in cell cycle arrest. HDAC inhibitors are thought to increase the rate of transcription of p21 by propagating the hyperacetylated state of histones in the region of the p21 gene, thereby making the gene accessible to transcriptional machinery. Further, non-histone proteins involved in the regulation of cell death and cell-cycle also undergo lysine acetylation and deacetylation by HDACs and histone acetyl transferase (HATs).
This evidence supports the use of HDAC inhibitors in treating various types of cancers. For example, vorinostat (suberoylanilide hydroxamic acid (SAHA)) has been approved by the FDA to treat cutaneous T-cell lymphoma and is being investigated for the treatment of solid and hematological tumors. Further, other HDAC inhibitors are in development for the treatment of acute myelogenous leukemia, Hodgkin's disease, myelodysplastic syndromes and solid tumor cancers.
HDAC inhibitors also have been shown to inhibit pro-inflammatory cytokines, such as those involved in autoimmune and inflammatory disorders due to their ability to inhibit the expression of pro-inflammatory cytokines such as TNF-alpha. For example, the HDAC inhibitor MS275 was shown to slow disease progression and joint destruction in collagen-induced arthritis in rat and mouse models. Other HDAC inhibitors have been shown to have efficacy in treating or ameliorating inflammatory disorders or conditions in in vivo models or tests for disorders such as Crohn's disease, colitis, and airway inflammation and hyper-responsiveness. HDAC inhibitors have also been shown to ameliorate spinal cord inflammation, demyelination, and neuronal and axonal loss in experimental autoimmune encephalomyelitis.
HDACs are divided into four classes. Class I is represented by yeast RPD3-like proteins (HDAC-1, -2, -3, and -8). Class IIa (HDAC-4, -5, -7, and -9) and class IIb (HDAC-6 and -10) share domains with yeast HDAC-1. Class IV (e.g., HDAC-11) shares properties of both class I and II HDACs. HDACs are zinc dependent deacetylases. In general, HDAC inhibitors normally include a Zn-binding group, as well as a surface recognition domain. There remains a need to develop new HDAC inhibitors, which will be useful in the treatment of various neurological or inflammatory conditions.
Hence, there is a need to develop new HDAC inhibitors, which will be useful in the treatment of various neurological or inflammatory conditions.